1) Field of the Invention
The present invention relates to a sealing material comprising a glass having a low melting point and a filler, which is suitable for sealing electronic parts such as semiconductor devices, fluorescent display tubes, laser diodes, and the like, and more particularly to a sealing material comprising a glass having a low melting point and a filler which is surface-coated with ZnO for improvement of the compatibility with low melting point glass.
2) Description of the Prior Art
Conventionally, the following types of glass with a low melting point hereinafter referred to as "low-melting-point glass") are commonly known: a PbO-B.sub.2 O.sub.3 based glass consisting essentially of 40 to 90 wt. % of PbO and 8 to 15 wt. % of B.sub.2 O.sub.3 ; a PbO-ZnO-B.sub.2 O.sub.3 based glass consisting essentially of 70 to 85 wt. % of PbO, 0.5 to 15 wt. % of ZnO, and 7 to 20 wt. % of B.sub.2 O.sub.3 ; and a PbO-B.sub.2 O.sub.3 -SiO.sub.2 based glass consisting essentially of 40 to 80 wt. % of PbO, 8 to 20 wt. % of B.sub.2 O.sub.3 and 5 to 45 wt. % of SiO.sub.2. Generally, the thermal expansion coefficients of these types of the low-meltingpoint glass are 90.times.10.sup.31 7 /.degree. C., and when such a low-melting-point glass is used to seal a material with a thermal expansion coefficient in the range of 40 to 80.times.10.sup.31 7 /.degree. C., a high stress is produced in the low-melting-point glass, so that such problems as cracking or breakage are caused. A sealing material in which the filler with low expansion characteristics is mixed with the low melting point glass is therefore mainly used to adjust the thermal expansion coefficient of glass to that of the material to be sealed.
Various types of materials exist which can be used as the filler with low expansion characteristics. Generally, zircon, willemite, cordierite, lead titanate, .beta.-eucryptite, tin oxide and mullite are used.
There are other types of materials with low expansion characteristics which have characteristics superior to those of the materials usually used as fillers. For example, silica glass has a lower dielectric constant (.epsilon.) than commonly used fillers. Therefore it is a material which possesses desirable characteristics in satisfying the recent trend that the materials used in semiconductor circuits to cope with high speed computation processes should have low dielectric constants. Also, in comparison with commonly used fillers, the use of crystalline oxides such as ZrW.sub.2 O.sub.8 has a considerable effect in lowering the thermal expansion coefficient of sealing materials, and the use of nitrides has a considerable effect in improving the mechanical strength of sealing materials.
As outlined above, silica glass, crystalline oxides such as ZrW.sub.2 O.sub.8, and the nitrides have the characteristics which are superior to those of commonly used fillers. However, a problem exists inasmuch as it is difficult to use these materials as fillers in actual practice because they are completely incompatible with low melting point glass, or their use is limited to small quantities. Specifically, when silica glass is dissolved into low melting point glass, the viscosity of that low melting point glass is drastically increased and the flow characteristics of the resulting sealing material are deteriorated. When a crystalline oxide such as ZrW.sub.2 O.sub.8 is dissolved into a low melting point glass, the glass has a strong tendency to crystallize and the flow characteristics of the resulting sealing material are deteriorated. Nitrides react with low melting point glass so that bubbles are readily formed in sealing materials.
In U.S. patent application 4,310,598, there is disclosed a sealing composition having a low expansion coefficient comprising a low-melting-point glass and .beta.-eucryptlte particles coated with tin oxide. The tin oxide coating on the .beta.-eucryptite particles, however, is not for improving the compatibility with low melting point glass, but for preventing the .beta.-eucryptite particles from being dissolved into the low-melting-point glass, and for reducing the deterioration of the insulating properties of the sealing composition.
Furthermore, in U.S. Pat. No. 4,352,889, there is disclosed a sealing composition having a low expansion coefficient comprising (i) a powdery coating material, specifically .beta.-eucryptite, with a negative thermal expansion coefficient, coated with titanium oxide and/or zirconium oxide and (ii) low-melting-point glass. The coating of the powdery coating material with titanium oxide and/or zirconium is not for improving the compatibility with glass, elther, but for preventing Li contained in .beta.-eucryptite from being diffused into glass and for reducing the deterioration of the insulating properties of the sealing composition.